After a lot of trial and error I have finally found an easy way to get bluetooth music and hands free phone in a 98 LS400 with Satnav (UK Model). This method involves no soldering whatsoever and is plug and play.
You need to buy a Dension Gateway Lite BT, as shown here: http://www.dension.com/products/car-multimedia-solutions/gateway-lite-bt. I paid about £90 for main off eBay.
You need need to plug the supplied three-way harness into the back of the CD changer and into the wiring loom that was plugged into the CD changer. The third plug plugs into the Dension Gateway Lite BT, I put mine in the glovebox as it also had a USB port and a line in that may be handy.
Instructions on how to get to the CD changer are here:
You then need to put some blank audio CDs in the changer to fool the system into thinking it is playing a CD.
That's it, wire in the microphone somewhere and pair your phone with bluetooth. You can no now listen to music, skip tracks and answer and end calls all with your stereo controls, all wirelessly and it sounds great! The phone audio quality is also very good.
Disclaimer- this works great on my car but don't blame me if it does not work for yours! The CD display indicator flashes on the EMV but otherwise it plays and acts normally. I'm also not 100% sure the bass speaker on the rear parcel shelfs works, but I'm not sure it ever did as I never used the stereo before installing this kit. This was tested on an iPhone 6. If you need a blank CD making message me! Good luck.
Updated! Although this works well the volume is not as loud as a CD and there is no bass from the sub, I am guessing that the signal is not in phase so some modification of the Dension outputs might be needed.